The results of studies of the pollution removal performance of constructed wetlands for urban runoff treatment are discussed. The design of urban runoff wetlands including primary treatment components, substrate composition and flow rate regulation are considered.
The use of front-end treatment systems such as oil interceptors, spillage containment facilities and wetland forebays or lagoons are recommended to provide an initial pollutant removal capacity as well as attenuating the impact of high discharges. Design criteria which will ensure inlet flow velocities do not exceed 0.7m/s and which will achieve optimum retention times (10-15 hours) for maximum pollutant removal are described. The relative advantages of different soil types are discussed and an overall minimum substrate depth of 0.6m is recommended for sub-surface systems. Two case studies of urban runoff wetlands in the UK which are currently being monitored are described.